Communications systems which transmit encoded motion picture data (an encoded stream) packetwise via communication networks such as IP networks etc., have been becoming widespread. As a communication protocol for transmitting multimedia data such as encoded streams etc., by packets, there is a protocol called RTP (Real-time Transport Protocol). RTP is used as a communication protocol on the upper layer protocol over UDP (User Datagram protocol) which is the communication protocol belonging to the layer higher than IP (Internet Protocol) layer.
A packet (RTP packet) in RTP is make up of as shown in FIG. 1 for example, of a RTP header portion and a RTP payload portion (e.g., see patent document 1). The RTP header portion includes a timestamp and a marker bit (M).
The timestamp indicates the time at which the packet (specifically, the first octet) is transmitted. The timestamp is used to schedule reconstruction of packets on the packet receiver side.
A marker bit indicates an important event (for example, the last packet of the encoded stream and the boundary of a frame) in the encoded stream. On the packet receiver side, the marker bit is used to determine the important event in the encoded stream.
The parameters (time and the value representing an event) of the timestamp and marker bit are calculated from the encoded stream.
As the schemes for sending encoding information (e.g., time resolution) of the encoded stream, there are two kinds; one is the scheme that transmits the encoding information together with the encoded stream and the other is the scheme that transmits the encoding information separately from the encoded stream. These schemes are described in patent document 2, for example. Here, the encoding information of the encoded stream is information related to the encoding of the encoded stream, e.g., time resolution.    Patent document 1: Japanese Patent Application Laid-open 2005-328131 (paragraph 0011, FIG. 2)    Patent document 2: Japanese Patent Application Laid-open 2006-13583 (paragraph 0007, FIG. 2).